1. Field of the Invention
This invention relates generally to projection displays and more particularly to head-mounted projection displays.
2. Description of the Related Art
Head Mounted Projection Displays (hereafter, HMPDs) have traditionally been, and are currently utilized in a variety of applications including maintenance, training, entertainment, and operations, including military operations. The term HMPD refers to a visual display that is mounted on a head or alternatively on, or inside a helmet with an image provided by an image source such as a liquid crystal display, a cathode ray tube or some other source being generally well known. The HMPD is positioned such that the displayed image is viewed as a real image by an eye. Furthermore HMPDs can be monocular or binocular.
In military applications, such as land combat ground soldier applications, requirements for target detection and recognition are very important. HMPDs seek to provide the user with enriched information, that is, information beyond that of what a human can gather with only biological vision.
Referring to FIG. 1, a typical land combat system for a ground soldier that exemplifies the need for a high resolution system with high field of view that includes an HMPD is illustrated at 10. The typical system includes a weapon subsystem 12, an integrated helmet assembly subsystem (IHAS) 14, computer master hub 16, radio 18, and associated interconnecting cable and hardware 20.
The weapon subsystem 12 typically includes an instrumented rifle having one or more sensors, for example, a weapon sight such as a thermal sight, video camera, and a laser rangefinder. As used in this specification the term sensor includes any input device for presentation on the HMPD. The exemplary monocular HMPD processes the data from the sensors and displays images of the data for presentation to the eye.
The IHAS 14 includes a helmet, preferably a light weight helmet and includes the HMPD for receiving and outputting processed sensor data. It is very important that HMPDs accurately receive and relay sensor data for object detection and recognition. For example, the Department of the Army specifies a capability objective for detection and recognition in its land warrior System.
The capability objective is defined in terms of performance, and the designer must account for the limitations of the human visual system, and the particular HMPD design for optimal detection and recognition. For example, HMPD design should recognize that peripheral vision is used to detect targets for pure detection, while central vision is focused on the recognition of the target. Therefore, the design of the HMPD represents complex tradeoffs that also include the size and weight of the HMPD components.
A typical HMPD and viewing system using a remote retro-reflector and method of displaying an image is described in U.S. Pat. No. 6,147,805 which is herein incorporated in its entirety by reference. The optics of the display include an image generator with light from the image generator transmitted through a lens and further transmitted to a beam splitter wherein the light is reflected from a retro-reflector to an eye. The retro-reflective screen is located external to the head mounted display.
Another head-mounted display is disclosed in U.S. Pat. No. 6,999,239 which is herein incorporated in its entirety by reference. The invention incorporates projective optics and a retro-reflective screen to eliminate the requisite use of an external retro-reflective screen to provide a see-through head mounted projection display. However, the optics is not optimized for detection and recognition capabilities, particularly since the see-through design decreases sensitivity for detection and recognition.
Referring to FIG. 2, an exemplary disclosure of an off-axis prism is illustrated as disclosed in U.S. Pat. No. 6,181,475. The prism can be considered as an off-axis eyepiece design for a nominal field-of-view (FOV) up to 40-degrees. The main disadvantage of the prism is the ability to achieve acceptable optical performance over a large FOV. The off-axis optical surfaces produce large amounts of aberrations such as astigmatism and distortion. In addition, chromatic aberration is induced to a single glass material. The performance of the prism can be improved by adding an aspheric surface in place of spherical surfaces to reduce the large off-axis aberration to achieve a reasonable optical performance, but the design still has limitations in correcting optical aberrations having only three working surfaces. While the off-axis prism provides a compact lens, detection and recognition capabilities are degraded by the aberration.
As can be appreciated, the application of the optimal HMPD display is complex. The design of the HMPD has a direct impact on human performance. Existing HMPDs are limited in their ability to support detection and recognition requirements, an especially important requirement in military ground operations or in aircraft, ship and harbor operations. These design limitations include, but are not limited to inadequate display resolution and field of view, limited graphics capabilities, and significant depth perception and peripheral vision problems. It is to be appreciated that once a particular HMPD is selected for use then the sensor that is to be associated with the application is selected so as to fully utilize the HMPD capabilities.